1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device equipped with plastic substrates, which has a light weight and can be prevented from breakage occurring through being dropped while in use.
2. Description of the Prior Art
As generally known in the art, a liquid crystal display (LCD) device is generally utilized as a screen for displaying information in a mobile terminal, following the development of wireless mobile telecommunication.
The liquid crystal display device employs a structure, in which a pair of glass substrates is generally joined together with liquid crystals being sealed there-between. However, recently it has become required that a substrate which is employed in a liquid crystal display device for a mobile terminal should be light in weight for ease in use, so that glass substrates have been changed to plastic substrates.
Highly polymerized compounds with a heat resistant property for temperatures up to 150 to 200° C., such as polycarbonate, polyimide, PES (polyether sulfone), PAR, PEN (polyethylene), and PET (polyether), are used as materials for plastic substrates.
A method for manufacturing a liquid crystal display device using a conventional plastic substrate is explained below with regard to FIGS. 1A to 1C.
Referring to FIG. 1A, a plastic substrate 10 with desired lower layer patterns (not shown), including a TFT and pixel electrodes, is provided. The plastic substrate is placed on a stage 1 with a vacuum hole 2, and fixed on the stage 1 with vacuum pressure.
Referring to FIG. 1B, an alignment material is coated on a fixed plastic substrate 10 by means of an offset printing method using a transfer plate 3. In FIG. 1B, the numeral 5 in the drawing refers to a coating roll.
Referring now to FIG. 1C, the coated alignment material is rubbed by a rubbing rag 6, to result in producing an alignment layer 11a on the substrate 10, which controls initial alignment of liquid crystal molecules. The numeral 7 in the drawing refers to a rubbing roll.
Next, although not shown in the drawings, lower substrate and upper substrate, to which alignment layers are formed through the above processes, are joined by sealing materials, and then liquid crystal molecules are filled in a space made between the substrates, resulting in the production of a liquid crystal display device incorporating plastic substrates.
However, according to the above explained prior art, a plastic substrate is transformed into a non-homogeneous shape owing to a curling property of the substrate, which occurs during fixture to the stage, and the vacuum pressure applied to fix the substrate. As stated above, a plastic substrate curls up away from the stage in undesired place including an edge thereof.
Accordingly, as the applied pressure is different in different regions owing to variation in height of the surface of the plastic substrate, the alignment materials are coated non-uniformly, and also the alignment layer, which is produced by rubbing of the alignment materials, is subjected to non-uniform rubbing over the whole substrate owing to variation of the friction between the rubbing rags and the alignment materials, produced by the vacuum pressure applied in the rubbing process and the non-uniformly coated alignment materials.
Further, as the non-uniform rubbing of the alignment layer produces a non-uniformity in cell gaps, the picture quality of a liquid crystal display device is reduced owing to the non-uniform cell gap.
Conventionally, to solve the above problems of the prior art, it had been proposed that the plastic substrate be fixed to the stage using a subsidiary substrate such as glass substrate and a thermal resistant tape.
However, although this prior art can prevent the plastic substrate from being non-desirously transformed, a non-uniformity of rubbing is produced in the fixing tape, which corresponds to the height of the surface of the tape, when the alignment materials are subjected to rubbing. As shown in FIG. 2, a non-uniformity of cell gaps is produced to a height of about 30 to 100 μm which corresponds to the height of the stacked fixing tape 26, when the plastic lower substrate 22 and upper substrate 24 are joined.
In FIG. 2, reference numeral 21 denotes a subsidiary substrate, 28 denotes a sealing material, d1 denotes the height of the stacked fixing tape 26, which is approximately 60 to 200 μm, and d2 denotes a cell gap, which is approximately 3 to 8 μm.
However, this technique can not finally solve the problems in the prior art whereby the picture quality of the liquid crystal display device is decreased due to the non-uniformity of cell gaps.